The present invention relates to a wavelength tunable optical filter for, rapidly and over a broad spectral band. The use of such optical filters is particularly advantageous in the field of optical telecommunications which use wavelength multiplexing, normally referred to as xe2x80x9cWDMxe2x80x9d (the acronym for the expression xe2x80x9cWavelength Division Multiplexingxe2x80x9d) or xe2x80x9coptical frequency division multiplexingxe2x80x9d.
Devices for selecting one or more optical frequencies from an input WDM signal are already known. However, none of the conventional tunable filter technologies based for example on the variation of a cavity length by moving a mechanical piece makes it possible to obtain switching times of less than a millisecond or at best a microsecond.
In fact, in the context of rapid frequency selection for switching optical packets requiring switching speeds situated in the nanosecond range, the only solution existing at the present time is based on frequency selection devices which use optical amplifiers utilized as open or closed optical gates.
A particular architecture of such a frequency selection device is described in U.S. patent application Ser. No. 2003/0002102, published on Jan. 2, 2003. In this document a description is in particular given of a device for selecting frequency division multiplex channels, an example of functioning of which, for a number of channels equal to N, respectively f1 to fN, is shown in FIG. 1.
Thus the channel selector according to D1 comprises a first 1xc3x97n cyclic demultiplexer Demux for demultiplexing the input multiplex as n interleaved frequency combs each consisting of m channels. The demultiplexer used is a 1 to n de-interleaving multiplexer based for example on filters of the Mach-Zehnder type, on etched gratings or on waveguide gratings of the AWG type (standing for xe2x80x9cArray Waveguide Gratingxe2x80x9d). A second mxc3x97m cyclic demultiplexer Demuxxe2x80x2 used as a router and consisting for example of an etched grating or a waveguide grating is provided for separating the channels of the interleaved combs. This second cyclic demultiplexer Demuxxe2x80x2 comprises n input ports connected respectively to the output ports of the first demultiplexer Demux by means of a first array composed of n optical switches Ii, with i between 1 and n. Each of the optical switches Ii composing the first array inserted between the two demultiplexers Demux and Demuxxe2x80x2 is advantageously formed by an optical amplifier. The frequency selector according to D1 also comprises a first mxc3x97m cyclic multiplexer Muxxe2x80x2 used as a router, the m input ports of which are connected respectively to the m output ports of the second mxc3x97m cyclic demultiplexer Demuxxe2x80x2 by means of a second array composed of m optical switches (formed by optical amplifiers) Ij, with j between 1 and m. The mxc3x97m cyclic multiplexer Muxxe2x80x2 is finally cascaded with a second nxc3x971 cyclic multiplexer Mux so as to recover the selected channel or channels on a single output port OF of the device.
In fact, depending on whether the optical switches composing the arrays of optical switches inserted respectively between the 1xc3x97n multiplexer Demux and the mxc3x97m demultiplexer Demuxxe2x80x2 and between the mxc3x97m demultiplexer Demuxxe2x80x2 and the mxc3x97m multiplexer Muxxe2x80x2 are switched on or off, it is possible to select and route the required frequency fi (1xe2x89xa6ixe2x89xa6N) of the input multiplex intended for the single output port OF of the device.
In order to obtain the required functioning of the device, the numbers n and m must be prime with each other (and N=nxc3x97m, with n less than m) and the components Demux, Demuxxe2x80x2, Mux and Muxxe2x80x2 must be designed to function with the same spectral spacing between channels xcex94f.
The configuration of the frequency selector illustrated in FIG. 1 therefore makes it possible to implement the selection of the required frequency amongst N channels by means of the activation of a first optical amplifier amongst the n making up the first array and a second optical amplifier amongst the m making up the second array.
Thus, in the frequency selection device of FIG. 1, the values of the frequencies processed are predetermined by the multiplexers and demultiplexers used, which ensures stability in terms of frequency, no tuning having to be carried out.
In addition, the device of FIG. 1 ensures the rapid selection of frequency. This is because the switching elements used, typically optical amplifiers, are elements which are intrinsically rapid since switching times of around a nanosecond can be achieved by such elements.
However, this solution has drawbacks. This is because, even if the architecture of the frequency selector according to FIG. 1 makes it possible to dispense with the need for using as many active elements, in this case optical amplifiers, as there are channels to be processed, it nevertheless involves using a consequent number of active elements. The device in FIG. 1 consequently occupies a large amount of space (for example a surface area of around 4.5xc3x974.5 mm2 for a conventional 16-channel frequency selector).
Equally it is necessary to provide as many electronic control elements as there are active elements. This is because, the activated optical amplifier not always being the same according to the frequency which it is wished to select, it is necessary to integrate in the device an electronic control chip for each of the optical amplifiers used. This involves in addition providing the fitting of many electrical connections and the necessary power supplies. Thus the use of the component in FIG. 1 will require a complex electronic card of large size.
Consequently one object of the present invention is to mitigate the above-mentioned drawbacks by proposing a rapidly tunable optical filter, that is to say one with very short tuning times, over a wide range of optical frequencies, in order to precisely obtain any one of the frequencies in the ITU (International Telecommunication Union) grid with a small spacing between consecutive frequencies of 50 or 100 GHz, and this by acting on only one control quantity, thus affording simplified control electronics.
The invention aims in particular to propose an optical filter combining the advantages set out above, whilst being very compact.
To this end, the invention makes provision for using first of all a Fabry-Perot cavity, that is to say a region delimited by two opposite reflective elements which are not selective in terms of wavelength and whose resonant modes are adjustable by electro-optical effect. Typically, an electrical field is applied to a PIN waveguide junction whose effective index is modified, according to the value of the electrical field, by virtue of a Franz-Keldish electro-optical effect or a quantum confined Stark electro-optical effect. The optical length of the Fabry-Perot cavity can thus be adjusted so as to be able to obtain the optical frequency values corresponding to the resonant modes of the Fabry-Perot cavity over the entire adjustment range xcex94fT required.
The Fabry-Perot cavity is optically coupled to an external reflector having a reflectivity which is selective in terms of frequency. This reflector consists for example of a sampled Bragg grating waveguide (xe2x80x9cSGWxe2x80x9d, standing for xe2x80x9cSampled Grating Waveguidexe2x80x9d). The sampled Bragg grating waveguide can be photo-written in a fiber (xe2x80x9cSFBGxe2x80x9d, standing for xe2x80x9cSampled Fiber Bragg Gratingxe2x80x9d), but any other waveguide can be used, in particular silica planar circuits or devices based on polymers. The sampled grating is designed so as to have N transmission peaks over the entire adjustment range mentioned above.
Such a device therefore comprises a first Fabry-Perot cavity with an operating mode adjustable by an electro-optical effect which can be controlled, coupled to a second external cavity.
Its operating principle is as follows. The optical frequency able to be transmitted by this set of two cavities coupled to each other is the frequency of the resonant mode of the Fabry-Perot cavity which best coincides with that of one of the transmission peaks of the external reflector. By changing the resonance conditions of the first cavity by acting on a control voltage determining the value of the electrical field in the phase tuning part, a sliding of the comb of the resonant modes of the first cavity causes a change in coincidences between on the one hand the optical frequencies of the resonance comb of the cavity and on the other hand the optical frequencies of the comb of the transmission peaks. Any new coincidence causes a mode jump and a transmission window then shifts to the new resonant frequency which coincides with another frequency value corresponding to a transmission peak of the sampled Bragg grating waveguide.
More precisely, the invention concerns a wavelength tunable optical filter, comprising a Fabry-Perot resonant cavity delimited by two opposite reflective elements not wavelength selective, and a reflector external to the said cavity, characterized in that the said external reflector exhibits transmission peaks for an integer number N of optical frequencies, and in that the said Fabry-Perot cavity delimits a phase tuning section and is sized so that the difference between the optical frequencies of any two resonant modes of the said cavity is never equal to the difference between the optical frequencies of any two transmission peaks of the said external reflector.
Thus, the resonant cavity being delimited by reflective elements not selected for wavelength, the cavity is strictly of the xe2x80x9cFabry-Perotxe2x80x9d type. The difference between the frequencies of any adjacent resonant modes is then practically a constant imposed essentially by the compositions and dimensions of the elements constituting the cavity. However, by varying its optical length by means of the phase tuning section, a sliding of the comb of the resonant frequencies of this cavity is caused.
The reflectors delimiting the cavity not being totally reflective, a wave will be able to pass through the entire cavity and external reflector if the latter has a transmission peak which coincides with one of the resonant modes of the cavity. As the cavity is designed so that the difference between the optical frequencies of any two resonant modes is never equal to the difference between the optical frequencies of any two transmission peaks of the reflector, a simple adjustment of the phase tuning section makes it possible to selectively make only one of the frequencies corresponding to the transmission peaks coincide with one of the resonant frequencies of the cavity and it is this single coincident frequency which will be transmitted.
In the particular case, important in practice, where the difference between any two adjacent optical frequencies of transmission peaks is constant, provision will also advantageously be made for the optical frequencies of the transmission peaks to be interleaved with consecutive optical frequencies of resonant modes. This arrangement, still obtained by an appropriate sizing of the Fabry-Perot cavity, permits adjustment ranges where the frequency value (or wavelength value) selected is a monotonic function of the voltage applied. This therefore simplifies the control of the phase tuning section.
Preferably, in the latter case, the ratio of the difference between two adjacent optical frequencies of resonant modes to the difference between two adjacent optical frequencies of transmission peaks will be chosen so as to be equal to N/(Nxe2x88x921). This arrangement ensures that the change from one frequency value selected to the following takes place by a sliding of the comb of the resonant modes according to a constant period. The result is an identical selectivity over the entire adjustment range.
In a preferred embodiment, the external reflector is a waveguide comprising at least one sampled Bragg reflective grating optically coupled to the first cavity.